Bright Lights, Big City

The first figure, an image of North America's lights at night, immediately suggested another analogy that can be used in discussing dark matter.

The authors say that dark matter is "the infrastructure of the cosmos." Focusing on Georgia and Florida in the first figure, it is quite simple to trace the route of Interstate 75 south from Knoxville through Georgia and into Florida, even though the highway itself is not visible. One might imagine the interstate highway system as the infrastructure that holds these cities and towns together.

Admittedly, the cities and towns preceded the modern highway system, but they grew based on early roads, railroads and trade routes that substantially parallel the highway.